A frequency synthesizer is a circuit that receives a reference signal at a reference frequency and generates an output signal at an output frequency. The output frequency may be related to the reference frequency by an integer ratio or a non-integer ratio, depending on the desired output frequency and the given reference frequency.
Frequency synthesizers are commonly used in various electronics devices. For example, a wireless device such as a cellular phone may include a frequency synthesizer to generate a local oscillator (LO) signal used for frequency downconversion or upconversion. A frequency synthesizer of a phase locked loop (PLL) type includes controlled oscillators such as voltage controlled oscillators (VCOs). An output of the VCO is often provided to a radio frequency (RF) mixer or to a microprocessor clock system. The frequency synthesizer controls the VCO such that its frequency or period is approximately equal to that of a stable frequency reference multiplied by a predetermined ratio. Coarse tuning systems compensate for variations in the VCO's frequency tuning range or center frequency by performing a coarse tuning of the tunable elements in the VCO. It is desirable to have as short as possible PLL coarse tune time in PLL normal operation.